Latin Name: Onopordum acanthium
Physical Description: it is a biennial herb from a taproot. The stems are erect, simple, branched, spiny-winged, and woolly and grow up to 3 meters tall. The woolly grey-green leaves are alternate, large and irregularly lobed, with sharp yellow spikes. Rosette leaves will grow to be 60 cm long and 30 cm wide. Flower heads are large (2 to 8 cm in diameter), globe-shaped, vibrant purple and grow at the end of leafy stalks, as either a single flower or as a cluster of flowers.
Habitat: it prefers moist soils but is drought tolerant. It can commonly be found growing in disturbed areas such as dry roadsides, rangeland, pasture, forest clearings, gravelly riverbanks and well-drained sandy or gravelly soils.
Impacts: it outcompetes native species and forms dense stands, decreasing habitat and forage availability. The dense stands also create a spiny barrier preventing wildlife and livestock movement. The sharp spines on leaves can cause injury to foraging animal’s mouths.
Reproduction: it reproduces solely by seed. Each plant can produce 8,400 – 40,000 seeds that can remain viable for up to 40 years. The seeds contain a substance that prevents them from germinating without sufficient water, so seeds can remain dormant in dry soil until the wet season, causing a large explosion of the population in the spring and fall.
Mechanical: small infestations can be effectively controlled by digging as long as the root is severed below the soil surface. Mowing before the flowering stage can reduce seed production, with repeated mowing throughout to growing season to control plants in differing maturity stages. Tilling will control emerged plants but will stimulate germination of existing seeds in the soil.
Chemical: Several herbicides with the following active ingredients can control Scotch thistle: Glyphosate, 2, 4-D, aminopyralid, clopyralid, picloram, dicamba, chlorsulfuron, and metsulfuron. For available products, contact your local agri-supply store. Prior to any herbicide application, read and follow the label instructions.
Biological: None available.